The behavior of fluid-air interfaces in a strong electric field has been of interest since Zeleny first observed the deformation of a liquid interface under the influence of a large applied voltage. He reported the formation of a cone with a fine thread of liquid coming from the apex and the disintegration of the thread into small droplets after a short distance. Taylor in 1964 was the first to propose a concise analytical model for the formation and structure of this electrified cone, and it is to him the name ‘Taylor Cone’ is attributed. When Taylor applied a field on the order of thousands of volts normal to the surface of the liquid, he also observed the formation of a conical liquid interface where a narrow jet of liquid droplets was emitted from the apex. This phenomenon has since become referred to as ‘electrospray’.
Using a cone as the equilibrium shape, Taylor recognized that both surface tension and electric stress must vary with the inverse of the radius of the cone. Using the potential for a cone as determined by Hall, Taylor reported an equilibrium expression for the electrified cone and calculated only one possible angle where equilibrium exists.
Sujatha et al. later approached the equilibrium of an electrified interface using the variational principle. Their paper was critical of Taylor's equilibrium model, noting that the excess pressure term is omitted in his formulation. Sujatha et al. found that there was no cone of any angle that satisfied their equilibrium expressions.
Deviations between measured cone angles and Taylor's predicted angle are addressed by Fernandez de la Mora, who accounts for the space charge in the emitted jet when predicting the shape of the interface. Fernandez de la Mora and Loscertales and Ganan-Calvo et al. report a study of the spray current and emitted droplet size of a conical electrified interface, and introduced scaling laws to predict these two quantities. Cloupeau and Prunet-Foch investigated different spraying modes (interface shapes) of a charged interface and Suvorov and Zubarev studied the evolution of Taylor cone formation for a liquid metal ion source. The latter predicted that the free surface approaches a conical shape with a semi-angle nearly identical to that calculated by Taylor.
Understanding the equilibrium of an electrified interface and the conditions required for: 1) the onset of an electrospray and 2) maintaining a steady electrospray once it is formed have important applications in a number of areas. Most notably, the use of electrospray revolutionized the field of mass spectrometry; a result of the seminal work presented by Fenn et al. Other applications of electrosprays include formation of thin films and colloid thrusters for propulsion.
Accordingly, it would advantageous to provide an electrospray emitter that can be easily manufactured and easily used. Further it would be advantageous to provide an electrospray emitter that can be used in the field to collect samples.